Well... For that resolution, let me make it clear from the beginning, dual 6870's are way way overkill for that.
Now that i have that off my chest (lol) let me answer your questions:
1.- Even a 990X will give you a bottleneck, so the question actually is: "will it be noticeable the lack of horse power from my PhII?". Nope, not at all. Most monitors are capped at 60Hz for vertical sync, so if you game at 99999FPS you'll just be seeing 60FPS (with v-sync on) or a lot of crippled images popping on the screen (tearing). And the PhII (which can hit 3.9Ghz easily, BTW) at 3.6Ghz is plenty to give you solid 60FPS+ on almost every game with the 2x6870's.
2.- If I recall it correctly, the 6870 has 2 6-pin molex'es, so it's more than 7A per card. I'd say that you need a PSU that can give at least 30A on the 12v rail, 15A per card. That's usually the 550W+ territory of PSU's.
This is from the AMD page: "500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin connectors for AMD CrossFireX™ technology in dual mode)"